It is known that antenna performance is dependent on the size, shape and material composition of the antenna elements, the interaction between elements and the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These physical and electrical characteristics determine several antenna operational parameters, including input impedance, gain, directivity, signal polarization, resonant frequency, bandwidth and radiation pattern. Since the antenna is an integral element of a signal receive and transmit path of a communications device, antenna performance directly affects device performance.
Generally, an operable antenna should have a minimum physical antenna dimension on the order of a half wavelength (or a multiple thereof) of the operating frequency to limit energy dissipated in resistive losses and maximize transmitted or received energy. Due to the effect of a ground plane image, a quarter wavelength antenna (or odd integer multiples thereof) operative above a ground plane exhibits properties similar to a half wavelength antenna. Communications device product designers prefer an efficient antenna that is capable of wide bandwidth and/or multiple frequency band operation, electrically matched (e.g., impedance matching) to the transmitting and receiving components of the communications system, and operable in multiple modes (e.g., selectable signal polarizations and selectable radiation patterns).
The half-wavelength dipole antenna is commonly used in many applications. The radiation pattern is the familiar donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical gain is about 2.15 dBi.
The quarter-wavelength monopole antenna disposed above a ground plane is derived from the half-wavelength dipole. The physical antenna length is a quarter-wavelength, but interaction of the electromagnetic energy with the ground plane (creating an image antenna) causes the antenna to exhibit half-wavelength dipole performance. Thus, the radiation pattern for a monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
The common free space (i.e., not above ground plane) loop antenna (with a diameter of approximately one-third the wavelength of the transmitted or received frequency) also displays the familiar donut radiation pattern along the radial axis, with a gain of approximately 3.1 dBi. At 1900 MHz, this antenna has a diameter of about 2 inches. The typical loop antenna input impedance is 50 ohms, providing good matching characteristics to the standard 50 ohm transmission line.
The well-known patch antenna provides directional hemispherical coverage with a gain of approximately 4.7 dBi. Although small compared to a quarter or half wavelength antenna, the patch antenna has a relatively narrow bandwidth. The small size is only attributable to the velocity of propagation associated with the dielectric material used between the plates of the patch antenna.
Given the advantageous performance of quarter and half wavelength antennas, conventional antennas are typically constructed so that the antenna length is on the order of a quarter wavelength of the radiating frequency and the antenna is operated over a ground plane, or the antenna length is a half wavelength without employing a ground plane. These dimensions allow the antenna to be easily excited and operated at or near a resonant frequency (where the resonant frequency (f) is determined according to the equation c=λf, where c is the speed of light and λ is the wavelength of the electromagnetic radiation). Half and quarter wavelength antennas limit energy dissipated in resistive losses and maximize the transmitted energy. But as the operational frequency increases/decreases, the operational wavelength decreases/increases and the antenna element dimensions proportionally decrease/increase. In particular, as the resonant frequency of the received or transmitted signal decreases, the dimensions of the quarter wavelength and half wavelength antenna proportionally increase. The resulting larger antenna, even at a quarter wavelength, may not be suitable for use with certain communications devices, especially portable and personal communications devices intended to be carried by a user. Since these antennas tend to be larger than the communications device, they are typically mounted with a portion of the antenna protruding from the communications device and thus are susceptible to breakage.
The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less obtrusive, and more efficient antennas that are capable of wide bandwidth or multiple frequency-band operation, and/or operation in multiple modes (i.e., selectable radiation patterns or selectable signal polarizations). For example, operation in multiple frequency bands may be required for operation of the communications device with multiple communications systems or signal protocols within different frequency bands. For example, a cellular telephone system transmitter/receiver and a global positioning system receiver operate in different frequency bands using different signal protocols. Operation of the device in multiple countries also requires multiple frequency band operation since communications frequencies are not commonly assigned in different countries.
Smaller packaging of state-of-the-art communications devices, such as personal communications handsets, does not provide sufficient space for the conventional quarter and half wavelength antenna elements. Physically smaller antennas operable in the frequency bands of interest (i.e., exhibiting multiple resonant frequencies and/or wide bandwidth to cover all operating frequencies of the communications device) and providing the other desired antenna-operating properties (input impedance, radiation pattern, signal polarizations, etc.) are especially sought after.
As is known to those skilled in the art, there is a direct relationship between physical antenna size and antenna gain, at least with respect to a single-element antenna, according to the relationship: gain=(βR)^2+2βR, where R is the radius of the sphere containing the antenna and β is the propagation factor. Increased gain thus requires a physically larger antenna, while users continue to demand physically smaller handsets that in turn require smaller antennas. As a further constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennas capable of efficient multi-band and/or wide bandwidth operation to allow the communications device to access various wireless services operating within different frequency bands or such services operating over wide bandwidths. Finally, gain is limited by the known relationship between the antenna operating frequency and the effective antenna electrical length (expressed in wavelengths). That is, the antenna gain is constant for all quarter wavelength antennas of a specific geometry i.e., at that operating frequency where the effective antenna length is a quarter of a wavelength of the operating frequency.
To overcome the antenna size limitations imposed by handset and personal communications devices, antenna designers have turned to the use of so-called slow wave structures where the structure's physical dimensions are not equal to the effective electrical dimensions. Recall that the effective antenna dimensions should be on the order of a half wavelength (or a quarter wavelength above a ground plane) to achieve the beneficial radiating and low loss properties discussed above. Generally, a slow-wave structure is defined as one in which the phase velocity of the traveling wave is less than the free space velocity of light. The wave velocity (c) is the product of the wavelength and the frequency and takes into account the material permittivity and permeability, i.e., c/((sqrt(εr)sqrt(μr))=λf. Since the frequency does not change during propagation through a slow wave structure, if the wave travels slower (i.e., the phase velocity is lower) than the speed of light, the wavelength within the structure is lower than the free space wavelength. The slow-wave structure de-couples the conventional relationship between physical length, resonant frequency and wavelength.
Since the phase velocity of a wave propagating in a slow-wave structure is less than the free space velocity of light, the effective electrical length of these structures is greater than the effective electrical length of a structure propagating a wave at the speed of light. The resulting resonant frequency for the slow-wave structure is correspondingly increased. Thus if two structures are to operate at the same resonant frequency, as a half-wave dipole, for instance, then the structure propagating a slow wave will be physically smaller than the structure propagating a wave at the speed of light. Such slow wave structures can be used as antenna elements or as antenna radiating structures.
As designers of portable communications devices (e.g., cellular handsets) continue to shrink device size while offering more operating features, the requirements for antenna performance become more stringent. Achieving the next level of performance for such communications devices requires smaller antennas with improved performance, especially with respect to radiation efficiency. Currently, designers struggle to obtain adequate multi-band antenna performance for the multi-band features of the devices. But as is known, efficiency and bandwidth are related and a design trade-off is therefore required. Designers can optimize performance in one (or in some cases more than one) operating frequency band, but usually must compromises the efficiency or bandwidth to achieve adequate performance in two or more bands simultaneously. However, most portable communications devices seldom require operation in more than one band at any given time.
In addition, modern portable communications devices must maintain size compactness and high efficiency while still attempting to provide adequate operating time with a limited battery resource. Antenna compactness and efficiency are therefore crucial to achieving commercially viable wireless devices.
The known Chu-Harrington relationship relates the size and bandwidth of an antenna. Generally, as the size decreases the antenna bandwidth also decreases. But to the contrary, as the capabilities of handset communications devices expand to provide for higher data rates and the reception of bandwidth intensive information (e.g., streaming video), the antenna bandwidth must be increased.
Current wireless communications devices operating according to the various common communications signal protocols, e.g., GSM, EDGE, CDMA, Bluetooth. 802.11x and, UWB and WCDMA, suffer operating deficiencies as set forth below.                A. Poor power amplifier (PA) efficiency due to sub-optimal PA load impedance (where the antenna impedance is the PA load impedance) as the PA's output power changes during operation of the communications device and as the antenna impedance change as the signal frequency changes.        B. Poor PA efficiency as set forth in A. above as further affected by the antenna's relatively narrow bandwidth due its relatively small size to fit within the available space envelope of the communications device (i.e., the Chu-Harrington limitation).        C. Poor PA efficiency due to a sub-optimal PA load impedance as the hand-effect or proximity effect detunes the antenna resonant frequency and/or modifies the antenna impedance.        D. Loss of radiative energy transfer (coupling efficiency) due to a sub-optimal PA output impedance (i.e., a sub-optimal antenna impedance) due to the use of a relatively small antenna and it corresponding relatively narrow bandwidth.        E. Loss of radiative energy transfer (coupling efficiency) due to detuning of the antenna resonant frequency caused by the hand-effect or proximity effect.        F. Poor PA efficiency due to impedance transformation to a higher value (i.e., 50 ohms) versus a lower value closer to the natural radiation resistance of the antenna.        
The teachings of the present invention are intended to overcome one or more of these disadvantages and thereby improve operation of the communications device.